The present invention relates to a method of manufacturing an optical disc and an optical disc device, and particularly is applicable to an optical disc on which serial data such as positional information, etc. are recorded by providing a groove with a guide groove for a laser beam. According to the present invention, when the serial data are phase-modulated and then frequency-modulated, the phase modulation is performed so that a period of logic "1" and a period of logic "0" are set to be equal to each other in modulated signals based on the phase modulation which correspond to the first half portion and the last half portion of each bit of the serial data respectively, thereby generating clocks with high precision.
In an optical disc, positional information on a laser beam, time information, etc. (hereinafter referred to as "wobble data") are detected on the basis of a groove serving as a guide groove for the laser beam. That is, in a manufacturing process for this type of the optical disc, a laser beam is illuminated (directed) onto a master disc while the master disc is rotated at a predetermined rotational speed. The illuminating position of the laser beam is successively displaced to the outer circumference of the master disc. Accordingly, during the manufacturing process, the master disc is exposed to the laser beam to form a track from the inner circumference to the outer circumference of the master disc in a spiral configuration.
In the optical disc manufacturing process as described above, developing and electro-forming steps are performed, and then a stamper is formed on the basis of the master disc. Thereafter, an optical disc is formed on the basis of the stamper. Accordingly, the optical disc is provided with a spiral groove extending from the inner circumference to the outer (peripheral) circumference in conformity with the illumination of the laser beam.
In the optical disc manufacturing process, when the master disc is exposed to the laser beam as described above, a reference signal synchronized with a predetermined carrier signal is frequency divided to generate a clock CK (see FIG. 1B). Further, a first reference signal synchronized with the clock and a second reference signal being a 1/2 frequency-divided signal of the clock CK are arranged in accordance with the logic level of wobble data ADIP (see FIG. 1A) respectively, whereby the wobble data is subjected to bi-phase mark modulation (see FIGS. 1A, 1B and 1C). Further, a synchronous pattern is inserted into the serial data generated by the bi-phase mark modulation as described above to form a channel signal (ch), and then a carrier signal used to generate the clock CK is frequency-modulated by the channel signal (ch) to form a modulated signal WB (hereinafter referred to as "wobble signal"). In the optical disc manufacturing process, the illuminating position of the laser beam is displaced in the radial direction of the master disc while following the signal level of the wobble signal WB.
Accordingly, a groove is formed on this type of the optical disc so as to be meandered (traced) in accordance with the synchronous pattern and the wobble data, and a spindle motor is controlled for rotation at a predetermined rotational speed so that the center frequency is set to a predetermined frequency. The wobble data are detected on the basis of the groove so that the recording/reproducing position can be checked. Further, reference clocks for various processing operations can be generated on the basis of the groove (see FIGS. 2A to 2C).
However, the optical disc as described above has a disadvantage that a clock cannot be generated with high precision from an area where the wobble data are recorded. That is, representing the 1/2 frequency of the clock CK by (ch) and representing the frequency of the carrier signal by n [Hz/ch], n+d [Hz/ch] is allocated to the logic "1" of the channel signal (ch) and n-d [Hz/ch] is allocated to the logic "0" of the channel signal (ch) to generate the wobble signal WB.
In order to simplify the description, assuming that n=4 and d=1/16, when the wobble signal WB crosses zero at the start time t0 of the synchronous pattern (FIG. 1A), DSV (Digital Sum Value) is set to zero in the synchronous pattern, whereby the wobble signal WB can cross zero at the end time t1 of the synchronous pattern.
However, at the subsequent falling time t2 of the clock CK, the wobble signal is kept at the frequency n+d [Hz/ch] for only one period of the clock CK, and thus the zero cross timing of the wobble signal WB is varied by only 2p/16 period.
Further, at the subsequent falling time t3 of the clock CK and at the additional subsequent time points t4 and t5, the wobble signal is kept at the frequency of n+d [Hz/ch] for only half a period of the clock CK, and then kept at the frequency of n-d [Hz/ch] for a subsequent half period, whereby the cross timing of the wobble signal WB is kept varied by only 2p/16 period.
At the subsequent falling time t6 of the clock CK, the wobble signal has the frequency n-d [Hz/ch] by only one period of the clock CK, whereby the phase variation from the time point t1 to the time point t2 is canceled, and the wobble signal WB crosses zero.
Accordingly, in the wobble signal WB which follows the groove, the zero-cross timing is varied relative to the clock CK. Thus even when the wobble signal WB is reproduced by detecting the groove, it is difficult to generate a high-precision clock from the reproduced wobble signal WB.
There may be considered a method of detecting the timing at which the wobble signal WB crosses zero, and locking a PLL circuit on the basis of this timing to generate a high-precision clock. However, in practice it is difficult to generate the high-precision clock because the clock varies in accordance with the content of the wobble data based on the timing at which the wobble signal WB crosses zero.